This invention relates to ablation devices that are used to create lesions in tissue. More particularly, this invention relates to ablation devices that use temperature-sensing elements to monitor the transmurality of the lesions.
The action of the heart is known to depend on electrical signals within the heart tissue. Occasionally, these electrical signals do not function properly. The maze procedure is a surgical operation for patients with atrial fibrillation that is resistant to medical treatment. In this procedure, incisions are created in the right and left atria to produce an orderly passage of the electrical impulse from the SA node to the atrioventricular node. Blind passageways are also created to suppress reentry cycles. Currently, the lesions may still be created using a traditional cut and sew technique. The scar tissue resulting from the procedure results in a non-conductive lesion.
Ablation of cardiac conduction pathways in the region of tissue where the signals are malfunctioning is now being used to replace the surgical incisions. Ablation is also used therapeutically with other organ tissue, such as the lungs, liver, prostate and uterus. Ablation may also be used in treatment of disorders such as tumors, cancers or undesirable growth.
Currently, electrophysiology (EP) ablation devices generally have one or more electrodes at their tips. These may be used for both diagnosis and therapy. In one instance, electrodes at the tips of EP ablation devices allow the physician to measure electrical signals along the surface of the heart. This is called mapping. When necessary, in another instance, the physician can also ablate certain tissues using, typically, radio frequency (RF) energy conducted to one or more ablation electrodes.
Sometimes ablation is necessary only at discrete positions along the tissue. This is the case, for example, when ablating accessory pathways, such as in Wolff-Parkinson-White syndrome or AV nodal reentrant tachycardias. At other times, however, ablation is desired along a line, called linear ablation. This is the case for atrial fibrillation, where the aim is to reduce the total mass of contiguous (electrically connected) atrial tissue below a threshold believed to be critical for sustaining multiple reentrant wavelets. Linear lesions are created between electrically non-conductive anatomic landmarks to reduce the contiguous atrial mass.
Linear ablation is currently accomplished in one of several ways. One way is to position the tip portion of the ablation device so that an ablation electrode is located at one end of the target site. This may be done, for example, with an electrode positioned on a xe2x80x9cpen-likexe2x80x9d device. Then energy is applied to the electrode to ablate the tissue adjacent to the electrode. The tip portion of the electrode is then slid along the tissue to a new position and then the ablation process is repeated. This is sometimes referred to as the xe2x80x9cspot burnxe2x80x9d technique. This technique is time-consuming (which is not good for the patient) and requires multiple accurate placements of the electrode (which may be difficult for the physician). Furthermore, even if the ablation process creates a continuously linear line along the top surface of the target tissue, it is not assured that the tissue is continuously and completely ablated through further layers of the target tissue (i.e. it is not assured that transmurality is achieved.) Transmurality is achieved when the full thickness of the target tissue is ablated.
A second way of accomplishing linear ablation is to use an ablation device having a series of spaced-apart band or coil electrodes which, after the electrode portion of the ablation device has been properly positioned, are energized simultaneously or one at a time to create the desired lesion. If the electrodes are close enough together the lesions run together sufficiently to create a continuous linear lesion. While this technique eliminates some of the problems associated with the xe2x80x9cspot burnxe2x80x9d technique, some repositioning of the ablation device may be required to create an adequately long lesion. In addition, it may be difficult to obtain adequate tissue contact pressure for each electrode in a multi-electrode ablation device. Also, the use of multiple electrodes to create the linear lesion tends to make the tip portion more expensive to make, more bulky and may cause the tip portion to be stiffer than is possible when a single, or very few, electrodes are used. The added complications resulting from the use of multiple ablation electrodes can also reduce overall reliability.
Ablation devices typically include a conductive tip, which serves as one electrode in an electrical circuit. The electrical circuit is completed via a grounding electrode that may also be on the device or may be coupled to the patient. By controlling the level of energy transmitted to the ablation electrode, the user is able to control the amount of heat generated. The ablation site may also be irrigated to cool the electrode and create greater lesion depth.
In order to control the level of energy transmitted, the user must monitor the level of energy being transmitted from the electrode. Typical systems for monitoring ablation energy rely on a thermocouple element located within the ablation device, generally near the electrode. This temperature-measuring element effectively measures the temperature of the electrode rather than the tissue being ablated. Particularly when the site is being irrigated with a conductive fluid, the temperature of the tissue may differ to some degree from the temperature of the ablation device.
Another concern with the ablation approaches is the difficulty of assessing when the lesion is transmural, that is, assessing that the lesion penetrates across the full thickness of the atrial tissue. Physicians have generally relied on their best judgment or historical data to predict when a lesion is fully transmural. Currently, there is no assessment of lesion transmurality. A physician simply creates a lesion by applying energy for a pre-determined length of time over a specific length (i.e. 30 W for 30 seconds over a length of 1 cm). This combination has been determined by exhaustive bench and animal experiments. Nonetheless, the human factor can result in moving the device too quickly, or changes in tissue thickness can require additional energy. If a lesion is incomplete, it may not be effective in controlling the arrhythmia, and may even be pro-arrhythmic.
It would thus be desirable to have an ablation device which, when positioned, is capable of easily and thoroughly creating a transmural lesion. It would further be desirable to have an ablation device that provides feedback that a lesion is complete and transmural. It would further be desirable to have a system for assessing the transmurality of lesions created by ablation, particularly to provide feedback to the user on the condition of the lesion while the ablation is taking place.
One aspect of the invention provides a sensor which monitors tissue temperature generated by an ablation apparatus on organic tissue. The sensor includes a temperature-sensing pad and an output device in communication with the pad. The output device receives and displays a representation of a lesion found on the ablated organic tissue. The temperature-sensing pad may incorporate temperature-sensing elements such as, for example, thermocouples, thermisters, temperature-sensing liquid crystals, or temperature-sensing chemicals. The temperature-sensing pad may be mounted on a glove, or may otherwise be adapted to fit over a user""s finger. The temperature-sensing pad may be mounted on a handle or stick, or other maneuvering mechanism or means for placing or positioning the pad against tissue. The apparatus may also include a conductive element. The output device of the sensor may be a visual display on a monitor or a visual display on the pad itself.
Another aspect of the present invention provides a system for assessing transmurality of an ablation in a tissue. The system includes an ablation apparatus which is used to ablate the front side of the tissue, a temperature-sensing pad which is placed against the back side of the tissue and senses temperature changes along the back side of the tissue. The system also includes an output device in communication with the pad that may indicate the temperature of the tissue. The system may also include temperature-sensing elements incorporated into the temperature-sensing pad.
Another aspect of the invention provides a method of ablating organic tissue. A sensor that senses the temperature of the tissue to be ablated is provided. The sensor is positioned against a back side of the tissue. The tissue is ablated with an ablation apparatus against the front side of the tissue. The pad senses the temperature changes as the tissue is ablated. The tissue may then be ablated in accordance with the temperature changes. The temperature of the tissue may be indicated using an output device.
Another aspect of the invention provides a device for monitoring temperature generated by an ablation apparatus on organic tissue. The device includes a temperature-sensing pad incorporating a plurality of temperature-sensing liquid crystals and an output device in communication with the pad that receives and displays a representation of a lesion found on the organic tissue. This pad may be used in conjunction with a color scheme in which each color indicates a temperature.
Another aspect of the invention provides a device for monitoring temperature generated by an ablation apparatus on organic tissue. The device includes a finger pad with plurality of temperature-sensing elements incorporated therein. The finger pad is operatively adapted to fit over a finger. The device also includes an output device in communication with the finger pad that is adapted to receive and display a representation of a lesion found on the organic tissue. The finger pad may also indicate the temperature generated by the ablation apparatus.
Another aspect of the invention provides a system for assessing transmurality of an ablation in a tissue. The system includes an ablation apparatus that is adapted to ablate a front face of the tissue and a temperature-sensing pad that incorporates a conductive element and senses temperature along a back side of the tissue. The system also includes an output device in communication with a pad which indicates the temperature of the tissue. The temperature pad may also be the conductive element.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.